Plants are dioecious with the male and female flowers borne on separate individuals. The flowers are solitary in the leaf axils. The female flowers produce ovoid seeds 2.0 to 4.5 mm long that have 3 to 4-angled areolae that are irregularly arranged (http://el.erdc.usace.army.mil/pmis/plants/html/najas_ma.html).

Flowers 1 per axil, staminate and pistillate on different plants. Staminate flowers in distal to proximal axils, 1.7-3 mm; involucral beaks 2-lobed, 0.3-0.7 mm; anthers 4-loculed, 1.7-3 mm. Pistillate flowers in distal to proximal axils, 2.5-5.7 mm; styles 1.2-1.7 mm; stigmas 3-lobed. Seeds not recurved, reddish brown, ovoid, 2.2-4.5 ´ 1.2-2.2 mm, apex with style situated at center; testa dull, 10-15 cell layers thick, pitted; areoles irregularly arranged, not in distinctive rows, not ladderlike, 3-4-angled, longer than broad, end walls slightly raised. 2n = 12 (Europe).

With its prickly internodes and prickles along the abaxial surface of the leaves, Najas marina is the easiest of our Najas to recognize.

Plants are reported to reproduce by seed and fragmentation (Tarver et al. 1986). Studies by Vierssen (1982) have shown seed germination of N. marina to be best in decomposing organic matter, at 24° C under dark conditions. It is in flower from September to November, and the seeds ripen from September to November. The plant prefers light (sandy), medium (loamy) and heavy (clay) soils. The plant prefers acid, neutral and basic (alkaline) soils and can grow in saline soil. It can grow in semi-shade (light woodland) or no shade.

Agami and Waisel (1986) found the germination of N. marina to be increased after passing through the digestive tract of mallard ducks, and postulate that ducks may be a major factor in long range dispersal of N. marina.

This species is currently considered non-native in GLANSIS, based on Mills et al. (1993) paper, which states, “Spiny naiad, a plant preferring to grow in brackish and alkaline waters, was first found in North America in 1864 in central New York's Onondaga Lake near Salina, New York (Stuckey 1985). The plants were growing near a salt mine in brackish water. Soon after this initial record, the plant was discovered in other areas of central New York. Spiny naiad is also known from the western Great Lakes region where it invaded in the 1930s. Fossil records of this plant from the midwest indicate that it was present in North America prior to glaciation, supporting debate about whether the newly discovered populations were indigenous or non-native. Two interpretations of the plant's distribution in the Great Lakes have been outlined by Stuckey (1985). He theorizes that the plant was pushed south during glaciation and reinvaded glacial lakes when the ice receded. He suggests that the species persisted in areas where the habitat remained favorable and reinvaded some areas, such as the western Great Lakes region, more recently. The introduction of the plant from Europe or another region where it is common in habitats made brackish and alkaline by human activities (such as areas around salt mines) is also possible. Central New York was a very active botanical center in 1864 and the possibility that the plant was overlooked for years is unlikely. The area around Onondaga Lake has been industrialized since the early 1800s when humans began developing the salt resources around the lake. The salt from this area was transported into other parts of the United States and the salt industry had the power to instigate the construction of the Erie Canal (Murphy 1978). We consider the introduction of spiny naiad into the industrialized area around Onondaga Lake to be a more likely scenario than the persistence of preglacial populations. Spiny naiad is now also known from Europe, Asia, Africa, Australia, South America, and Central America (Stuckey 1985).”

The classification of this species is a source of ongoing contention in Great Lakes states, and some environmental managers consider it to be naturalized in the region, though they note that its range appears to slowly be expanding (Nault, M., 2017, pers. comm). Its cosmopolitan nature and sporadic distribution around the world is likely due to its very specific environmental needs, which include brackish, highly alkaline waters. No active management is currently being conducted on N. marina in Wisconsin, and further research is needed to determine its ultimate status in the Great Lakes.